Bedsores, or decubitus ulcers, can be a serious problem in bedridden or wheelchair-bound patients, particularly for people who are paralyzed, emaciated, post-surgical, elderly, or diabetic. The ulcers frequently penetrate through not only the skin, but the underlying muscle and bone as well. With the serious infections that often ensue, pressure ulcers can become life-threatening.
As the elderly population increases with demographic trends, the incidence is likely to increase. The results of the last National Pressure Ulcer Surveys from 1989 to 1997 indicate that despite the growth in the wound care and therapeutic surface industries, the incidence of pressure ulcers appears to have increased over this period. It is clear that while new treatment solutions may be relatively effective, their cost precludes their use by the vast majority of caregivers in the settings in which pressure ulcers and other chronic wounds must be managed. Disproportionately, this includes the nursing home, home care, and of course, the overseas markets where resources are limited. The consensus among thought leaders in the international medical community supports the contention that less expensive medical solutions are required generally and urgently. The invention to be described here is intended to fulfill this societal need.
Bedsores, or pressure ulcers, were named because they most commonly develop where tissue pressures are greatest—over the bony prominences, such as the heels, sacrum (tailbone), ischia, greater trochanters, and ankles (external malleoli). At these sites where the pressure on the skin is concentrated, blood flow can be restricted. If nutrient deficit exceeds tissue demand over a given interval, the tissue will start to die locally, resulting in an ulcer.
It is generally recognized that it is important to limit both skin warming and moisture accumulation to effectively combat skin breakdown. This has been embraced by professional bodies and recognized thought-leaders in the wound care medical community.
The normal core temperature of the human body is between 36° and 38° C. Skin temperature typically ranges between about 30° C. and about 34° C., depending on ambient temperature, the amount and type of clothing being worn, the core temperature, and where the skin is located on the body. However, on a typical mattress, seat cushion, seat back, etc., heat is trapped between the body and the covered skin surface, and the skin temperature rises rapidly to and may reach 35 to 37 degrees C. This small temperature elevation that occurs with the skin in contact with the mattress, seat cushion, etc., has important physiologic effects.
When a patch of skin is warmed beyond a specific level referred to as the “perspiration threshold” of approximately 32 to 34° C., local perspiration in the region increases markedly. The accompanying moisture softens the skin (maceration), which makes it more susceptible to breakdown. The build-up of moisture increases the friction between the skin and the surface materials resulting in increased shear stresses in the tissue. It has also been shown that elevated skin temperature is associated with increased metabolic demand, therefore, researchers have speculated that elevated skin temperature increases the susceptibility of the tissue to ischemic injury, particularly when both nutrient supply and metabolite removal are reduced by loading. Generally, tissue metabolic rates increase by approximately 10% for each one degree Celsius increase in temperature. Warmed tissue generates an increased demand for blood supply that can be met when the skin is not under significant load. At interface pressures of 20 or more mm Hg, as occur under the bony prominences on a mattress or seat, blood flow can not be increased to meet this demand, and the tissue becomes ischemic. Other research looked directly at tissue injury and temperature. One demonstrated that skin tissue with reduced blood supply has been shown to be less susceptible to injury when tissue temperatures were slightly reduced. In a second study, identical pressures were applied to the skin tissue of research animals at nearly 300 sites. The skin temperatures at the interface varied between 28 and 36 degrees C. The results showed a very strong positive correlation—nearly perfect, in fact—between skin temperature and degree of skin breakdown.
When skin temperatures are maintained within certain limits, the person or animal is more comfortable. For humans, comfort is optimal when the skin temperature is maintained close to its natural (non or lightly insulated) temperature of 30 to 34 degrees C., even when insulated support conditions are employed. The devices described herein have important medical and non-medical applications. The non-medical applications include most seating and bedding applications such as mattresses for the home, seating or seat backs for the office, home, and vehicle markets.
Steady State vs. Temporary Cooling
Limiting the warming of the skin that occurs when it is insulated during therapeutic support reduces the risk of bedsores, aids healing, and enhances comfort. In the prior art, skin cooling is accomplished using what is known as a “low-air-loss bed” (LAL), which may cost $40,000.00 or more. LAL beds utilize pumps or blowers to eject steady streams of air through small vents in the bladders of an air mattress. Air flowing across an underside of the ticking convectively removes heat that passes from the patient's body into the surface. Heat from the body is subsequently transported with the ejected air from the bed as it is continually cycled. Although some LAL surfaces are effective at providing steady state cooling, they require power that is typically provided by electro-mechanical means, such as motors. Accordingly, LAL surfaces may be noisy, require extensive engineering and operator training, and they may be imposing to both patient and caregiver. Additionally, they may increase the risk of bio-aerosol contamination, i.e., the risk of spreading germs in the hospital or nursing home environment.
Temporary skin cooling can be accomplished by increasing the heat input required to increase the temperature of the surface. The quantity of heat required to increase a temperature of a specific quantity of material by a specific temperature is called the specific heat. For example, the specific heat of a specific alloy of aluminum can be expressed in Joules/kg-degree K. The quantity of heat required to raise the temperature of a given body is referred to as the heat capacity of the body. If a large body and a small body are both made of the same material, for example, the larger body will have a greater heat capacity although both will have the same specific heat. A surface of high specific heat material such as silicone gel or fluid, or even a waterbed, will provide temporary cooling, because a great deal of the body's heat will flow from the skin, initially at approximately 30 to 34° C. to the surface, initially at 23° C. room temperature. Phase change material which, as it undergoes phase-change, tremendously increases the capacity of a material to absorb heat, while maintaining the same temperature. All such surfaces will initially feel quite cool to the user. Such approaches, however, only delay skin warming. A steady flux of heat into the surface will eventually cause all of the phase change material to change phase, and/or the high heat capacity material to warm. In order to provide continuous, steady-state cooling, the heat must be removed from the system and transferred to the environment or to another system that is external to the surface to be cooled. A need exists for non-powered, or, stated otherwise, self-powered devices to provide steady state cooling at the level of the expensive, externally powered LAL surfaces currently in use. It is particularly valuable to develop such a device that provides cooling without spreading airborne pathogens from the occupant's skin surface into the common environment.
It is important to note that materials of sufficiently high thermal conductivity, mechanical compliance, and relatively low cost did not exist for such an application until the last few years. Heat transfer by conduction has not generally been considered practical for use in applications in which heat transfer paths are large (greater than 10 cm) and temperature differences are small (less than 10° C.) between the region to be cooled and the environment. This is particularly true for biomedical applications.
The likelihood of bedsore formation is reduced by lowering tissue metabolic rate (and therefore reducing tissue ischemia in pressurized tissue with reduced blood flow) and limiting local perspiration, which weakens the outer skin layer (the stratum corneum) over time. These inventions may be used as an aid in the healing of early stage bedsores or other skin ulcers. Moderate cooling of the skin during support (from 35° C. to 37° C. down to the 30° C. to 34° C. range) also makes the user more comfortable. The proposed inventions, therefore, have not only medical applications, but applications in the general consumer niche as well.